There is known a reinforcing bar binding machine having a binding wire feed-mechanism for feeding out a binding wire and hanging the binding wire around a reinforcing bar and a binding wire twist mechanism for twisting the binding wire wound around the reinforcing bar to bind for carrying out binding operation of 1 cycle by successively executing binding ire binding operation and binding wire twisting operation by trigger operation. Further, as disclosed in, for example, JP-A-08-034406 (claim 2, paragraph No. 0022, paragraph No. 0055), there is proposed a reinforcing bar binding machine for pulling back a binding wire after binding wire feed operation, bringing a loop of the binding wire into close contact with a reinforcing bar to start twisting operation. According to the reinforcing bar binding machine, by pulling back the binding wire, a line length thereof is adjusted in accordance with a diameter of the reinforcing bar to thereby promote finishing and also save to reduce an amount of consuming the binding wire.
In the case of constituting the reinforcing bar binding machine such that the binding wire is pulled back after the binding wire feeding operation and the twisting operation is started in a state of bringing the binding wire into close contact with the reinforcing bar, setting of a pull back tension poses a problem. When a feeding force of the binding line feed mechanism is weak, time is taken in pulling back the binding wire or the binding wire cannot be pulled back. Further, when the feeding force is conversely excessively large, there is a concern of cutting the binding line.
As a means for preventing such a drawback from being brought about, it is conceivable to provide electric control means for setting a motor torque to a degree of being sufficient for pulling back the binding wire and stopping a motor by detecting an increase in a drive current by a rapid increase in a pull back resistance in finishing to pull back the binding wire. However, a flexibility of the binding wire significantly differs by a kind of the binding wire or an environmental condition of an outside air temperature or the like and a width of varying the pull back resistance is wide and therefore, a width of changing the current is not constant, a peak value of the current in stopping the binding wire is significantly varied also by a remaining capacity of a power source battery and therefore, it is difficult to control to stop the motor stably without being influenced by these factors. Further, when there is constructed a constitution of setting a threshold value of the current of stopping the motor at each time for pertinently pulling back the binding wire, the operation becomes complicated and practical performance is deteriorated. Further, there also poses a problem that cost is increased by providing a control circuit including a current detecting circuit or the like.